Media outside Westminster Magistrates' Court in London ahead of the appearance of Julian Assange

WikiLeaks founder Julian Assange has vowed to fight attempts to extradite him to Sweden (AP)

Wikileaks founder Julian Assange is driven into Westminster Magistrates Court in London after being arrested

WikiLeaks chief Julian Assange

Wikileaks founder Julian Assange

Visa has suspended all payments to WikiLeaks

Supporters of WikiLeaks founder Julian Assange say they fear that his extradition to Sweden for alleged sex crimes will pave the way for a future appearance in an American court.

Joe Lieberman, the head of the US Senate's Homeland Security Committee, claimed yesterday that the leaks organised by Julian Assange and his associates were "serious violations of the Espionage Act". US State Department spokesman Philip Crowley added: "What WikiLeaks has done is a crime under US law."

The US Justice Department is under intense political pressure, especially from the Republicans, to pursue Mr Assange, with leading figures of the right, such as Sarah Palin, using the leaks to attack the Obama administration.

"They have got to do something, otherwise they will face pretty savage criticism," said a European diplomat. "I suppose the best hope for the Americans is that he gets convicted in Sweden and serves a jail sentence there."

Although the US authorities have as yet made no official request for his extradition, informal discussions have been held between American and Swedish officials to that end. Mr Assange's London solicitor Mark Stephens said that being sent to Sweden from the UK would make his client extremely vulnerable. "His Swedish lawyer has said that it would be quite unsafe for Julian in Sweden at this time," Mr Stephens said. "Not in terms of he would be harmed in Sweden, but that Sweden is not the end game."

Eric Holder, the US Attorney General, has said that Justice Department lawyers are examining how Mr Assange could be indicted over the leak of the diplomatic cables. "This is not just sabre-rattling on our part," he said. "We are talking about one of the most serious violations of the Espionage Act in our history. To the extent that we can find anybody who was involved in the breaking of American law... they will be held responsible; they will be held accountable."

However, diplomatic sources say that it is not clear whether the US authorities will be able to prosecute Mr Assange under the Espionage Act. They point out that whereas it is illegal for government officials with security clearance to leak classified documents to WikiLeaks, it is not clear whether it is illegal for the website to make it public.

There is yet to be a successful prosecution of a third-party recipient of a leak and the possible acquittal of Mr Assange on such a charge would be doubly embarrassing for the US administration.

The Justice Department is, therefore, also considering bringing possible charges of Mr Assange receiving stolen government property. But that too could pose difficulties as journalists have in the past used leaked government documents in the US without being prosecuted.

Meanwhile it was announced yesterday that Mr Assange's legal team will be led by the prominent Australian-born barrister Geoffrey Robertson QC, who has flown back from Sydney to take on the case. Mr Robertson, who has been involved in a number of high-profile human rights cases, could take the appeal against the granting of extradition to Sweden all the way to the Supreme Court, with legal proceedings stretching into months.